How to measure it, when to retest, and what to do with the number. Cardiorespiratory fitness predicts how long people live better than most of what we measure in a standard panel, and almost nobody measures it. Here is how to bring VO2 max testing into clinical practice, which measurement to use for which patient, and when to retest.
On October 6 I hosted a webinar with Brooks Leitner, MD, PhD, founder of VO Health, on VO2 max as a clinical metric. Brooks spent three years at the NIH Clinical Center running VO2 max tests, DEXA scans and insulin clamps on healthy volunteers before his MD-PhD at Yale. The full session is below, and the one-page takeaway guide we made for it is at the end of this post. This post is the longer version: the evidence, the measurement options, and the three use cases Brooks laid out for a practice like the ones we work with.
Watch the full session: VO2 Max in Clinical Practice, Ultralight x VO Health, October 6, 2026
Fitness as a vital sign: why functional assessments matter
The American Heart Association said this plainly in 2016. Its scientific statement, titled "A Case for Fitness as a Clinical Vital Sign," called cardiorespiratory fitness "the only major risk factor not routinely assessed in clinical practice" and asked that improving it "become a standard part of clinical encounters" (Ross et al., Circulation 2016, retrieved 2026-10-08). The same authors restated it in a 2024 update: fitness should be "a routine assessment in all health care settings."
Cardiorespiratory fitness (CRF) describes how well the heart, lungs, circulation and muscles supply and use oxygen during sustained physical activity. It helps explain how much exertion a person can tolerate, and large studies show a strong association with longevity.
In a Cleveland Clinic study of 122,007 adults, the least fit quarter had approximately five times the adjusted mortality hazard of the elite group over a median follow-up of 8.4 years (Mandsager et al., JAMA Network Open 2018, retrieved 2026-10-08). The elite group represented roughly the top 2.3 percent for age and sex. The association remained after researchers accounted for factors including smoking, diabetes and coronary artery disease. For clinicians, this supports assessing fitness alongside those familiar risk factors.
The findings also suggest that differences in fitness matter well below the elite level. A 2024 review covering 199 cohort studies found that each 1-MET higher fitness level was associated with an 11 to 17 percent lower risk of death from any cause (Lang et al., BJSM 2024, retrieved 2026-10-08). MET stands for metabolic equivalent, a unit that expresses oxygen use relative to rest. One MET conventionally equals 3.5 milliliters of oxygen per kilogram of body weight per minute. A fitness level of 8 METs therefore represents the capacity to sustain an effort requiring about eight times resting oxygen use. These were observational findings: they do not establish that increasing an individual patient's fitness by 1 MET will reduce their risk by that exact percentage.
Brooks described why fitness testing caught his attention at the NIH, including in volunteers whose standard labs were normal:
"With a low cardiorespiratory fitness or low VO2 max, you could then just see that these are the people likely to develop some sort of chronic disease within the next, say, five years rather than 20 years."
His point is that low fitness can raise concerns about a patient's long-term health even when routine bloodwork looks reassuring. The specific timeline reflects his clinical interpretation; these studies do not establish when an individual patient will develop disease.
How fitness was assessed matters when interpreting these results. The Cleveland Clinic researchers estimated it from treadmill speed and incline. Other studies directly measured oxygen consumption during exercise or used different estimates. VO2 max measures the maximum amount of oxygen someone can use during intense exercise and is a direct measure of cardiorespiratory fitness. The broader research supports assessing fitness in clinical practice; each wearable or blood-based estimate still needs evidence showing how accurately it measures fitness and tracks changes over time.
What does VO2 max actually measure?
VO2 max is the most oxygen the body can use at peak effort, in milliliters per kilogram of body weight per minute. Brooks frames it with the oxygen cascade. Oxygen binds hemoglobin in the lungs, is pumped by the heart through the vessels to working muscle, and is consumed in the mitochondria. In his words:
"Oxygen literally comes from the environment through the heart, lung, blood vessels, blood, muscles, and mitochondria."
VO2 max helps explain why the same daily task can feel easy for one person and exhausting for another. Brooks used carrying groceries as an example: someone with very high fitness might use "maybe 20% of their peak capacity," while someone with very low fitness could need "up to 80% of their VO2 max." Those percentages illustrate the difference. As aerobic capacity falls, everyday activities require a larger share of what a person can do before reaching exhaustion.
At very low levels, this can affect someone's ability to live independently. A 2009 review by Roy Shephard suggested that independence becomes more difficult below a VO2 max of roughly 18 ml/kg/min for men and 15 ml/kg/min for women (BJSM 2009, retrieved 2026-10-08). These are approximate levels of concern, rather than cutoffs that determine whether someone can live alone. Strength, balance and other health conditions also affect independence. VO2 max is also one of the four tests that predict how your patients age.
Aerobic capacity generally declines with age, and the decline becomes faster in later life. The Baltimore Longitudinal Study of Aging followed 810 adults aged 21 to 87 (Fleg et al., Circulation 2005, retrieved 2026-10-08). Researchers estimated declines of 3 to 6 percent per decade in participants' 20s and 30s, rising to more than 20 percent per decade in their 70s and beyond. Brooks's rule of thumb, about 10 percent per decade after 30, is a useful simplification, but it does not describe every age or predict an individual patient's trajectory.
For a practice, the reason to establish a baseline in midlife is to track how that patient's fitness changes over time. Testing in their 40s can give the clinician a starting point for an exercise plan and later comparisons, while daily activities are still manageable.
How is VO2 max measured? Four options and what each costs
There is no single right method. The choice depends on the patient and the question.
Lab test (CPET on a metabolic cart)
- Measures: directly measured VO2 max, plus ventilatory thresholds.
- Accuracy: the reference standard. A study of test-retest variability reported variation of about 4 to 5 percent in healthy adults (Shephard et al., Eur J Appl Physiol 2004, retrieved 2026-10-08).
- Cost to the practice: a cart or portable analyzer, a trained technician, 30 to 40 minutes of patient time, and a maximal effort. Independent labs charge roughly $150 to $400 cash, with Fishtown Medicine's testing service offering one example (retrieved 2026-10-08).
- Limits: some patients need an adapted protocol or medical supervision, particularly those with significant deconditioning, cardiac conditions or mobility limitations. The effort involved can also discourage repeat testing.
Field tests (Cooper 12-minute run, walk tests)
- Measures: an estimate from distance covered. The original Cooper validation study, involving 115 Air Force men, found a correlation of 0.897 between the 12-minute run and lab-measured VO2 max (JAMA 1968, retrieved 2026-10-08).
- Cost: nearly zero.
- Limits: the Cooper run requires maximal effort, and its original validation population was young, fit and male. Walk tests use different protocols and may suit patients who cannot complete a maximal run.
Wearable estimates (Apple Watch, Garmin)
- Measures: an algorithmic estimate from heart rate and pace during outdoor walks or runs.
- Accuracy: a 2025 study of 30 Apple Watch users found that the watch underestimated lab VO2 max by 6.07 ml/kg/min on average, with a mean absolute percentage error of 13.3 percent (Lambe et al., PLOS ONE 2025, retrieved 2026-10-08). A 2025 validation study of the Garmin Forerunner 245 found errors of 7 to 8 percent overall and 9 to 10 percent in highly trained athletes (Eur J Appl Physiol 2025, retrieved 2026-10-08).
- Cost: nothing additional if the patient already wears one.
- Limits: useful for following trends, but individual estimates can differ substantially from lab results. Interpret them alongside the patient's symptoms, activity and other clinical findings. Our guide to integrating wearable data into the clinical chart covers how to use these numbers in practice.
Blood-based estimate (proteomics)
- Measures: a predicted VO2 max from circulating proteins, plus pathway scores.
- Accuracy: see the next section.
- Cost: a venous draw or a capillary self-collection device. No maximal effort.
- Limits: new. The published science is from 2021 and 2024; the commercial test launched this year.
Can a blood test measure VO2 max? How VO Health's estimate works
VO Health uses proteins in the blood to estimate VO2 max without an exercise test. This approach is called proteomics: measuring proteins to learn about biological processes in the body.
Published research supports the idea that blood proteins can provide information about fitness. A 2021 study identified 147 proteins associated with VO2 max (Robbins et al., Nature Metabolism 2021, retrieved 2026-10-08). A 2024 study involving 14,145 people developed and evaluated a protein-based fitness score across four cohorts (Perry et al., Nature Medicine 2024, retrieved 2026-10-08). These studies support further development of blood-based estimates, but they do not establish the accuracy of VO Health's commercial test.
Brooks explained that VO Health develops its estimate by comparing blood samples with directly measured VO2 max tests performed in the same week, usually on the same day. Collecting both close together reduces the chance that a change in fitness between tests will distort the comparison. According to Brooks, the test measures about 1,000 proteins and uses 50 to estimate VO2 max. The report also includes scores for ten biological pathways related to fitness, including fat metabolism, inflammation and oxidative stress.
How accurate is the estimate?
Brooks presented three performance figures during the webinar. These were company-reported results that had not yet been peer reviewed:
Agreement with measured VO2 max: an R-squared of 0.89. This describes how much of the variation in measured VO2 max the model accounted for in the data presented. It does not mean the test is "89 percent accurate."
Average prediction error: about 10 percent. This describes the average percentage difference between the blood estimate and the exercise-test result; an individual result could differ by more.
Repeatability: a reported difference of 0.9 percent between blood estimates collected two days apart in 14 people. This small study suggests that repeat samples can produce similar results. It does not establish how accurately the test detects changes after an exercise program.
For clinicians, the unresolved question is whether the blood estimate can reliably track meaningful changes in an individual patient's fitness. Published validation will help clarify that, including which patients the test works best for and how large a change needs to be before it should influence care. VO Health has registered a 250-person validation study (NCT07393841, retrieved 2026-10-08).
Why is repeat VO2 max testing hard, and does a blood test help?
I took the test myself. The Tasso collection device sits on the upper arm and collects a small blood sample in a few minutes. Patients can provide a sample without completing another maximal exercise test.
Brooks described why that matters even for highly trained athletes:
"It's actually even harder in elite athletes to get them to test. Often it doesn't fit into their training programs. They hate it. It's a really hard test."
Easier collection could make follow-up testing more practical. Whether those repeat results accurately reflect a patient's response to training still needs to be established.
How to use VO2 max testing in clinical practice: three use cases
This is the part of the session I most wanted clinicians to hear. Brooks organized clinical use into three questions, each with a suggested retesting schedule. These intervals reflect his approach rather than established guidelines for blood-based fitness testing.
1. Flag accelerated decline
Brooks described age-related fitness decline as roughly 1 percent per year after 30, with faster losses in later life. A sustained decline beyond what is expected for a patient's age warrants a closer look.
"If you have a patient that is declining three to five percent in a year and they do that two years in a row, this accelerated decline is something that could put them at risk not only for cardiovascular disease, but also issues of functional capacity."
For clinicians, the takeaway is to investigate a persistent downward trend rather than assume aging explains it. Changes in activity, weight or health may contribute. Compare results from the same testing method under similar conditions, since measurement variability can also account for small changes.
When to test: annually for general monitoring; quarterly if you suspect decline, such as after weight gain, reduced exercise or a new diagnosis.
2. Monitor the response to an exercise prescription
Brooks suggested using repeat testing to assess whether an exercise plan is improving fitness. His team tests the Yale women's rowing team every two weeks to study how quickly the protein-based estimate changes. For a patient following a three-month exercise program, he suggests monthly testing.
The clinical takeaway is to establish a baseline and assess the response over time. Interpret the result alongside the patient's exercise performance and symptoms. How reliably the blood estimate detects short-term training changes still needs validation.
When to test: monthly during a structured program, then back to annual monitoring.
3. Protect lean mass during weight loss
Brooks highlighted weight loss as another reason to follow fitness. VO2 max is commonly reported per kilogram of body weight, so the number can rise as weight falls, even without an improvement in aerobic capacity. He described an unchanged result as a reason to look more closely:
"If that number is not improving in response to weight loss, that would really suggest that you're losing active skeletal muscle or lean tissue that you really want to be keeping."
The clinical takeaway is to investigate, rather than diagnose muscle loss from VO2 max alone. An unchanged or falling result can prompt a review of body composition, strength and exercise capacity.
This is particularly relevant during GLP-1 treatment. A 2025 review in JCEM describes losses of fat-free mass during treatment and limited evidence about improvements in cardiorespiratory fitness (Liu et al., retrieved 2026-10-08). Fat-free mass includes muscle and other nonfat tissue, so these measurements should be interpreted together.
When to test: quarterly during active weight loss, for about a year.
Additional uses under investigation
Brooks also mentioned two special populations with work underway: a fit-for-surgery screen about 12 weeks before a planned procedure, followed by six to eight weeks of exercise; and cancer rehabilitation, where his group's forthcoming paper flags survivors at or below 16 ml/kg/min as the group at highest risk of falls. The preoperative literature supports the idea in principle: a systematic review of 37 studies supports the use of cardiopulmonary exercise testing (CPET), which measures oxygen use and other responses during exercise, to assess risk before certain abdominal surgeries (Moran et al., BJA 2016, retrieved 2026-10-08).
The prescription is fitness
The reason to measure is to act. Brooks's framing was that medicine runs on thresholds: high LDL prompts a statin, high A1c prompts metformin.
"What we're hoping to do is flag VO2max low, the prescription is fitness."
The prescription itself is well established. The WHO 2020 guidelines call for 150 to 300 minutes of moderate or 75 to 150 minutes of vigorous activity per week (Bull et al., BJSM 2020, retrieved 2026-10-08). On intensity, Brooks offered a rule of thumb from the FITT framework (frequency, intensity, time, type): the less time a patient has, the larger the share that should be high intensity. One hour a week, about half hard. Three to four hours, a quarter to a third hard. A 2015 meta-analysis found both interval and steady-state training raise VO2 max by about 5 ml/kg/min against controls, with a small extra edge for intervals (Milanović et al., Sports Medicine 2015, retrieved 2026-10-08).
What a blood estimate cannot tell you yet
Before adding the test to routine care, consider three limitations.
Its accuracy still needs published validation. The performance figures Brooks presented are company-reported results awaiting peer review. For decisions such as surgical risk assessment, the evidence supporting CPET should not be assumed to apply to a blood-based estimate.
The pathway scores need careful interpretation. The report provides ten pathway scores rather than individual results for the roughly 1,000 proteins measured. Brooks explained that much of the protein-level information does not yet have a clear clinical use:
"You don't need every single biomarker. You need the ones that really have outsized returns on the interventions that follow."
The practical question is whether a score supports a specific clinical decision. A pathway score alone does not establish a diagnosis or identify which intervention will help.
The test is still developing. Brooks said VO Health launched the test in July 2026. As the dataset grows, the company may update its prediction model. Record the test version alongside the result, where available, and check whether model changes affect comparisons over time.
Where this fits in our practices
Most clinicians we work with establish a baseline, recommend an intervention and reassess the patient's response. Fitness testing can support that process when the measurement is appropriate for the patient and changes are interpreted in context. It also gives clinicians a concrete way to discuss exercise capacity and progress with patients.
We built Ultralight for practices that follow patients over years, with lab results, wearable data and clinical notes in one chart. To see how fitness results and follow-up testing can fit into your longevity or functional medicine practice's workflow, book a walkthrough with our team.
For more information about the Molecular Fitness Blood Test, visit VO Health. Download our one-page VO2 Max Field Guide for a summary of the session, and subscribe to The Modern Clinician for weekly guidance on building a modern practice.
FAQ
How often should VO2 max be tested?
No guideline sets an interval. The cadence Brooks laid out: annually for general monitoring, quarterly if you suspect accelerated decline or during active weight loss, and monthly during a structured three-month exercise protocol.
Is VO2 max a vital sign?
Not officially. The American Heart Association's 2016 scientific statement and its 2024 update argue that cardiorespiratory fitness should be treated as one and assessed routinely. No payer or guideline body requires it today.
How much does VO2 max decline per decade?
About 10 percent per decade on average after 30, but the rate is not constant. The Baltimore Longitudinal Study of Aging found 3 to 6 percent per decade in the 20s and 30s and more than 20 percent per decade after 70.
How accurate is Apple Watch or Garmin VO2 max compared with a lab test?
Independent studies put Apple Watch error at roughly 13 to 16 percent, usually underestimating by 4 to 6 ml/kg/min, and Garmin at 7 to 10 percent. Good enough to watch a trend, not to make a clinical decision.
Can a blood test measure VO2 max?
It can estimate it. Proteomic models published in 2021 and 2024 correlate with lab VO2 max at about 0.7. VO Health's commercial test reports a higher figure (R-squared 0.89) that the company presented at our webinar and has not yet published in a peer-reviewed journal.
Do GLP-1 medications lower VO2 max?
Absolute aerobic capacity can fall as fat-free mass is lost; a 2025 review estimates 25 to 40 percent of GLP-1 weight loss is fat-free mass. VO2 max per kilogram should rise with weight loss. If it does not, suspect muscle loss and adjust the plan.
What CPT code is used for VO2 max testing?
CPET is billed under CPT 94621. Most longevity practices offer it cash-pay or inside a membership; independent labs charge roughly $150 to $400 per test.
Sources
All retrieved October 8, 2026.
- Ross R, et al. Importance of Assessing Cardiorespiratory Fitness in Clinical Practice: A Case for Fitness as a Clinical Vital Sign. Circulation 2016;134:e653-e699. https://www.ahajournals.org/doi/10.1161/CIR.0000000000000461
- Ross R, et al. Update to the 2016 AHA cardiorespiratory fitness statement. Prog Cardiovasc Dis 2024;83:10-15. https://doi.org/10.1016/j.pcad.2024.02.003
- Mandsager K, et al. Association of Cardiorespiratory Fitness With Long-term Mortality Among Adults Undergoing Exercise Treadmill Testing. JAMA Netw Open 2018;1(6):e183605. https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2707428
- Lang JJ, et al. Cardiorespiratory fitness is a strong and consistent predictor of morbidity and mortality: an overview of meta-analyses. Br J Sports Med 2024;58:556-566. https://bjsm.bmj.com/content/58/10/556
- Fleg JL, et al. Accelerated longitudinal decline of aerobic capacity in healthy older adults. Circulation 2005;112:674-682. https://doi.org/10.1161/CIRCULATIONAHA.105.545459
- Shephard RJ. Maximal oxygen intake and independence in old age. Br J Sports Med 2009;43:342-346. https://bjsm.bmj.com/content/43/5/342
- Shephard RJ, Rankinen T, Bouchard C. Test-retest errors and the apparent heterogeneity of training response. Eur J Appl Physiol 2004;91:199-203. https://link.springer.com/doi/10.1007/s00421-003-0990-4
- Cooper KH. A means of assessing maximal oxygen intake. JAMA 1968;203(3):201-204. https://jamanetwork.com/journals/jama/article-abstract/337382
- Lambe R, et al. Accuracy of Apple Watch VO2 max estimates. PLoS ONE 2025;20(5):e0323741. https://doi.org/10.1371/journal.pone.0323741
- Garmin Forerunner 245 validation. Eur J Appl Physiol 2025. https://doi.org/10.1007/s00421-025-05923-x
- Fishtown Medicine, VO2 max testing service page (cash price example). https://www.fishtownmedicine.com/services/vo2-max-testing-philadelphia
- Robbins JM, et al. Human plasma proteomic profiles indicative of cardiorespiratory fitness. Nat Metab 2021;3:786-797. https://research.bidmc.org/jeremy-robbins/publications/human-plasma-proteomic-profiles-indicative-cardiorespiratory-fitness
- Perry AS, et al. Proteomic analysis of cardiorespiratory fitness for prediction of mortality and multisystem disease risks. Nat Med 2024. https://doi.org/10.1038/s41591-024-03039-x
- Attia P. Proteomics and cardiorespiratory fitness. https://peterattiamd.com/proteomics-and-cardiorespiratory-fitness/
- Topol E. The Flawed V02 Max Craze. Ground Truths, Feb 23, 2026. https://erictopol.substack.com/p/the-flawed-v02-max-craze
- Nomic Bio and VO Health press release, Oct 6, 2026. https://www.financialcontent.com/article/bizwire-2026-10-6-nomics-proteomics-platform-to-power-vo-healths-molecular-fitness-insights
- VO Health MISFIT study, NCT07393841. https://ichgcp.net/clinical-trials-registry/NCT07393841
- Liu, Weeldreyer, Angadi, et al. GLP-1 receptor agonists, lean mass and cardiorespiratory fitness. JCEM 2025. https://pmc.ncbi.nlm.nih.gov/articles/PMC12448640/
- Moran J, et al. Role of cardiopulmonary exercise testing as a risk-assessment method in patients undergoing intra-abdominal surgery: a systematic review. Br J Anaesth 2016;116:177-191. https://doi.org/10.1093/bja/aev454
- Bull FC, et al. WHO 2020 guidelines on physical activity and sedentary behaviour. Br J Sports Med 2020;54:1451-1462. https://doi.org/10.1136/bjsports-2020-102955
- Milanović Z, Sporiš G, Weston M. Effectiveness of HIIT and continuous endurance training for VO2max improvements. Sports Med 2015;45:1469-1481. https://doi.org/10.1007/s40279-015-0365-0
- Webinar recording and transcript, Ultralight x VO Health, Oct 6, 2026 (all Brooks Leitner quotes). https://www.youtube.com/watch?v=4xBpuyoXlkU
